Great Heathen Army

The Great Heathen Army was a coalition of Viking warriors from Denmark, Sweden, and Norway led by Halfdan Whiteshirt of Jorvik, Bjorn Ironside of Svibjod, Ivar the Boneless of Sudreyjar, and Sigurdr Snake-in-the-eye of Sjaelland that invaded England in 865 to avenge the death of Ragnarr Lodbrok at the hands of Aella of Northumbria.

History
In 865, when King Ragnarr Lodbrok of Svibjod was thrown into a pit of snakes by King Aella of Northumbria, he is said to have exclaimed "how the piglets would grunt if they knew how the old boar suffered" before being killed by the snakes. These words proved prophetic, for his sons Halfdan Whiteshirt of Jorvik, Bjorn Ironside of Svibjod, Ivar the Boneless of Sudreyjar, and Sigurdr Snake-in-the-eye of Sjaelland gathered large armies and landed in Northmbria and conquered York in 867. This became the center of the Danish kingdom of Jorvik, and the Great Heathen Army made Jorvik their base of operations.

The Great Heathen Army then divided into multiple groups: King Halfdan's army headed south and invaded Mercia while his half-brother Ubbe Ragnarrsson led an army north to fight Northumbria. The other Viking armies also fanned out through England, defeating local armies and seizing several English castles. The Great Heathen Army destroyed Northumbria, killing King Aella in a torturous and gruesome way called the "Blood Eagle". They also smashed East Anglia to the southeast and also took over half of Mercia, while Wessex to the south prepared to resist.

The West Saxons of King Aethelred of Wessex were victorious at the Battle of Ashdown and the Battle of Englefield in 871, but they defeated Aethelred at Reading and then at Basing, where he died. The Great Heathen Army then threatened to capture his brother Alfred the Great at the royal manor of Chippenham, but Alfred escaped and mobilized the fyrd, the army of Wessex. At the Battle of Edington in 878, he destroyed King Guthrum of Jorvik's army, and this prevented the Great Heathen Army from seriously threatening him again.